Ion mobility spectrometry (IMS) is used in chemical and biological agent detectors and provides good sensitivity, low power requirements, and operation at atmospheric conditions. In general, IMS is a gas-phase ion separation technique that separates different chemical species as a function of both size, i.e. average collisional cross section area, and mass-to-charge ratio (m/z). The mass-to-charge ratio is the molecular weight of a species divided by the number of charges, which in many instances is one because the species is singly charged. In a typical IMS device, a sample to be analyzed is collected and passed through an inlet and into an ion source region where ions are formed. The ions then pass along a drift tube containing a potential gradient that is used to accelerate the ions against a counter-current drift gas, e.g. air. Under the influence of the accelerating voltage, the lighter and smaller ions, i.e. the ions having a smaller mass-to-charge ratio and average collisional cross section area, reach the detector first, and heavier and larger ions arrive later.
As the ions exit the drift tube, they collide with a detector or collector, for example a Faraday Cup. Since the ions exit the drift tube at different times, chemical species in the sample are identified based on known arrival times of certain ions at the detector. When a given ion or group of ions reach the detector, they create a voltage peak that is proportional to the number of ions striking the detector. These peaks are referred to as the IMS spectra, and the IMS spectra are averaged to increase the signal-to-noise ratio (SNR). Therefore, a time window for monitoring the detector is established and monitored for these peaks. In addition, a voltage threshold is established for each peak, and the number of peaks in excess of the voltage threshold is monitored. An alarm condition in the monitoring device is established for a given contaminant in the sample when a sufficient number of peaks above the voltage threshold that are associated with that contaminant are detected.
IMS begins by forming reactant ions through the interaction of reactant molecules with a radioactive source. The sample to be analyzed is ionized by gas phase interaction and subsequent chemical reaction with these reactant ions, creating product ions. The ionization mechanism is proton-transfer and electron-transfer. The reactant and product ions are exposed to an accelerating electric field that is maintained along the entire length of a drift tube. Radioactive sources are not pulsed, producing ions as a continuous stream. Conventional IMS devices use a gating grid upstream of the drift tube to modulate the ion beam in order to break the continuous stream of ions into discrete packets. Then the ion gate is switched to pulse and deflect the ions periodically. This periodic pulsing, however, results in a duty cycle for the IMS device of less than 1%, that is less than 1% of the product ions are utilized in sample analysis.
This short duty cycle and a variety of other operational factors, e.g. matrix interference, affect the sensitivity of the IMS device. For example, a duty cycle of less than 1% decreases the sensitivity of the device. In addition, a significant number of collisions and interactions can occur among the ions, producing clusters and polymers of ions. Ion clusters reduce sensitivity, lower the resolution of the IMS device and cause voltage peaks to appear broadened resulting in lowered resolution.
Conventional IMS devices are also not useful for the characterization of mixtures, because components present in high concentrations or components with high proton affinities will dominate the spectrum. Compounds with lower proton affinities only appeared in the plasmagram when higher concentrations of them are analyzed. Similarly, matrix components present together with the analyte in the sample may cause problems during analysis.
Therefore, an IMS system and method are needed that provide increased sensitivity and utilize significantly more of the sample product ions generated during ionization. In addition, the system and method would be suitable for very low concentrations of chemical compounds and could easily and reliably analyze binary, tertiary and higher order mixtures of chemical compounds.